As is well known in the art, the power steering system of an automobile is an apparatus for supplying steering oil to a power cylinder cooperating with the steering system by an oil pump driven by engine power so as to facilitate operating a steering wheel. The power steering system is designed in such a manner that large hydraulic pressure is produced when parking or driving a car at a low speed, whereas small hydraulic pressure is produced therein to secure safety when driving a car at a high speed.
An example of such a power steering system will be described with reference to FIGS. 1 and 2. Referring to these figures, a valve body 11 is mounted within a valve housing 10, and a port 12 and an oil groove 13 forming an oil passage for steering oil are formed on the outer surface of the valve body 11 so that they communicate with each other. An input shaft 20, which is connected to a steering column and rotated in response to the operation of the steering wheel, is mounted on the inner side of the valve body 11. A bore 21 is formed at the center of the input shaft 20, and a plurality of slots 22 are circunmferentially formed on the exterior of the input shaft 20 at equal intervals. Ports 23, which communicate with the port 12 of the valve body 11 to become the oil passage for the steering oil, are formed in the slots 22 of the input shaft 20, respectively. Further, a torsion bar 14 is mounted in the bore 21 of the input shaft 20 and is connected to a gear unit 15.
In the meantime, when a driver operates the steering wheel, the input shaft 20 connected to the steering column is rotated in response to the operating direction of the steering column so as to control the oil passage for the steering oil. Accordingly, the operation of the power cylinder is controlled, and thus, the steering of the car is performed. However, a shock is generated due to the sudden variation in and disturbance of flow of the steering oil which passes at high speed and pressure through the port 12 of the valve body 11 and the ports 23 of the input shaft 20 when the direction of rotation of the input shaft 20 is changed, while another shock is generated due to physical friction between the input shaft 20 and the steering oil. These shocks become sources of noise and vibration. Further, wear on the valve body 11 and the input shaft 20 is produced, and thus, the life of valve body 11 and the input shaft 20 is shortened. Accordingly, some problems involved with a reduction in reliability may be produced.
In order to reduce the hydrodynamic and mechanical shock produced in response to a change in the direction of the input shaft 20, the surface of the input shaft 20 should be precisely machined. Moreover, right and left ends of the slots 22 are chamfered so as to reduce fluid resistance exerted thereon. Chamfered faces at the right and left ends of the slots 22, i.e. chamfers 24 specifically shown in FIG. 2, become elements which greatly influence the reduction of fluid resistance. Thus, the chamfers 24 are precisely machined by using an edge-grinding machine.
In general, the machining precision of the chamfers of the input shaft for use in power steering systems is performed by means of a sampling inspection through a visual inspection of an inspector. However, visual inspections that relied entirely upon the determination of the inspector may vary greatly according to measuring errors inherent to the respective inspectors. Thus, there is a problem in that time and manpower are greatly consumed. In particular, the sampling inspection is involved with a problem in that reliability for all the input shafts cannot be completely guaranteed. Therefore, a total inspection for the input shafts is required. However, a machine for correctly and rapidly performing a total inspection for input shafts has not yet been developed and thus the total inspection of input shafts cannot be performed using the prior art.